It is recognized that the synthesis of erythropoietin (Epo) is tightly regulated by oxygen delivery to specific renal cells, providing an effective mechanism that maintains red cell mass. There is significant evidence, derived mostly from basic research and limited clinical observations, that Epo production is modulated by other hormonal mechanisms in addition to oxygen delivery. In particular, in vitro studies indicate that B-adrenoreceptor agonists and adenosine stimulate Epo production. The physiological significance of these findings, however, is not certain. Our specific aim is to determine if Epo production is blunted by B- adrenergic receptor blockade with propranolol or adenosine receptor blockade with caffeine (7).